1. Field of Invention
This invention relates to the field of portable communication transceivers and specifically to a portable transceiver circuit which determines the quality of a received signal and directs the tranceiver to select an alternative antenna if the signal quality deteriorates below a predetermined level.
2. Description of the Prior Art
Portable radio transceivers are used in several facets of communications technology. Portable transceivers can be found in paging systems, two-way communication systems such as those used by police and other public servants, and more recently portable receivers have found use in portable data terminals which are used to communicate with a host computer over a radio link. Portable data terminals provide computer diagnostics and expanded computer power in the portable terminal.
Typical portable paging receivers in the past have incorporated loop-type antennas which are typically responsive to the magnetic field component of a transmitted RF signal. This type of antenna system provides marginal performance, but is desirable because the loop antenna can be concealed within a radio housing and responds well when the receiver is in close proximity to a human body. However, the overall sensitivity of the antenna/receiver combination is somewhat degraded where the receiver is in close proximity to a human body. In addition, loop-type antennas exhibit certain problems which make them undesirable for use in a portable device which also utilizes a transmitter.
Portable units which contain both a receiver and transmitter are utilized in public service and private industry where communication between a number of units is desirable. The units typically contain a receiver and transmitter used in combination with an external helical, monopole or dipole antenna. These antennas are responsive to the electrical field of an RF signal and provide superior reception and transmission characteristics, when the antenna is located externally to the portable device housing. In addition, helical and monopole antenna performance is degraded substantially when the antenna is operated in close proximity to a human body.
A paging receiver is normally worn on the body when in use. Unlike the pager, a portable transceiver is utilized in many different environments. It may be handheld in front of the face, worn on the hip, or placed on a conducting or non-conducting surface.
With the advance of data processing and data communication techniques, it is now desirable to manufacture portable communication devices which are capable of transmitting and receiving data in cooperation with a host computer. These devices are for instance useful to servicemen who use the portable terminal to perform on-site computer testing and diagnostics, and to keep records thereof. It is desirable to manufacture a portable data terminal as small as possible, so the terminal can be transported easily. In addition, it is also desirable to provide a portable data terminal with an antenna system which is completely enclosed within the terminal housing. This feature enhances the portability of the data terminal, as well as providing protection for the terminal antenna system.
These terminals may be carried in the hand, worn on the hip, put in a coat pocket, used on a desk, or put into a service tool kit. The orientation of the unit, as well as its physical environment is constantly changing as it is utilized from one time to the next.
Since the portable data terminal is operated in a number of distinct environments and orientations, it is necessary to provide an antenna system which is adaptable to a changing environment. Adaptability is especially useful when the portable data terminal is used in close proximity to a human body.
In an adaptable antenna system, the receiver must include a means for alternately activating antennas in response to the quality of a received signal. Prior art antenna switching devices have typically based the antenna switching decision on the signal strength of an incoming RF signal. This technique switches antennas when the incoming signal drops below a predetermined threshold. To implement a signal strength based antenna decision, expensive RF circuitry is required to process signal strength information. In addition, signal strength based antenna switching is prone to "chattering" when the signal level for each antenna is below a predetermined threshold.
For the foregoing and other shortcomings, there is a need for an improved adaptable antenna system control scheme for use with a transceiver in which the antenna system activates a particular antenna based on the signal quality of the incoming signal. Signal quality information is derived from the receiver output signal and does not require any special RF amplifiers to determine signal quality.